A Cation-Captured Palladium(0) Anion: Synthesis, Structure, and Bonding of [PdBr(PPh3)2]- Ligated by an N-Heterocyclic Phosphenium Cation

Unsaturated N-heterocyclic phosphenium cations (uNHP) stabilize the [Pd-0(PR3)(2)X](-) anion proposed over the past decade to be the crucial but elusive intermediate in palladium-catalyzed cross-coupling reactions (X = halide). Insertion of metal into the P-Br bond of the precursor mesityl-substituted bromophosphine gives the structurally characterized Pd(0)-phosphenium complex (uNHP(Mes))Pd(PPh3)(2)Br, which features a long Pd-Br bond (2.7240(9) angstrom) and the shortest known Pd-P bond (2.1166(17) angstrom). The reaction is proposed to proceed by an associative pathway involving a Pd-bromophosphine complex that undergoes P-to-Pd bromide transfer.